1. Field of the Invention
The invention is a flame retardant for styrenic resins containing both aromatic bromine and an olefin. More specifically, the olefin contains an internal olefin instead of a terminal olefin.
2. Description of the Background Art
Styrenic resins or styrenes are well known synthetic organic polymers. Styrenic resins are thermoplastics and have excellent mechanical properties as well as good chemical resistance. The properties that make styrenes useful for many applications as solid polymers also make them very desirable as foamed polymers.
A number of processes have been developed over the last forty years to prepare styrenic foams for a variety of applications. Two particularly useful processes have been developed to make extruded polystyrene foam (XPS) and expanded polystyrene foam (EPS). Extruded polystyrene foam and expanded polystyrene foam are used in industry for a variety of commercial applications. Many of those applications require the foam to be flame retardant to meet specific standards.
The most commonly used materials to flame retard polymer resins are halogenated organic compounds and are well known in the literature to be highly effective flame retardants. Brominated organic compounds are widely used in styrenic resins and foams to provide ignition resistance.
Haloaromatic allyl ethers are known to be acceptable flame retardants for use with expanded polystyrene foam. In known examples, the allyl ether is typically a terminal olefin rather than an internal olefin. Golborn and Taylor studied these systems and proposed a mechanism for the flame retardant action. However, they did not report any examples with an internal olefin or postulate whether the mechanism is applicable to a compound with an internal olefin.
Allylic ethers with a terminal olefin are known to have flame retardant activity in styrenic foams such as expanded polystyrene foam. However, these compounds are not widely used in applications with expanded polystyrene foam. The primary reason that terminal olefin allyl ether compounds are not acceptable is their reactivity and the method by which expanded polystyrene foam is prepared. Expanded polystyrene foam is most commonly prepared by a one step polymerization of styrene monomer. In this method the flame retardant and other additives are added to styrene monomer prior to reaction, and then the styrene is polymerized in water to form beads. The additives, including the flame retardant, are present during the polymerization and are thus incorporated into the beads.
Olefins, other than the styrene monomer, often act as chain transfer agents during polymerization resulting in lower molecular weights of the polystyrene. Lower molecular weight polystyrene typically shows a decrease in the physical properties of the polystyrene. While not wishing to be bound by theory, it is hypothesized that terminal olefins are more readily accessible to participate as chain transfer agents in the polymerization reaction. Internal olefins are more sterically hindered and less accessible to the growing polymer chains and thus are less likely to act as chain transfer agents. Therefore, known allylic ethers with a terminal olefin are generally not considered suitable flame retardants for this application.
The compound 1,4-bis(2,4,6-tribromophenoxy)-2-butene and its flame retardant activity is acknowledged in U.S. Pat. Nos. 2,488,499 and 3,787,506. However, U.S. Pat. No. 3,787,506 states that the compound is not suitable for use in styrenic applications due to its low thermal stability. This compound can be prepared using either cis-1,4-dichloro-2-butene or trans-1,4-dichloro-2-butene. This stereochemistry is maintained during the reaction resulting in cis/trans isomers in the product. No mention is made in these patents to a specific isomer being used in the synthesis, nor is there any indication that the inventors recognized that different isomers have different properties such as thermal stability.
Hexabromocyclododecane (HBCD) in particular was discovered to be a suitable flame retardant for styrenic polymers in general and more specifically for styrenic foams. HBCD is a highly brominated aliphatic compound with an unusually high thermal stability. The high thermal stability and bromine content of HBCD result in excellent performance at low loading levels with a minimum effect on polymer properties. Thus, HBCD has become the material of choice with flame retarding styrenic foams. HBCD has the particularly important properties of high flame retardant efficiency at low concentrations and high thermal stability.
Other flame retardants have been proposed and tested in styrenic foams. Representative examples can be found in the following patent documents of DE1469819, DE2813872, U.S. Pat. No. 4,272,583, DE19630925, GB1182964, FR2138745, JP53008663, and JP57038831. For the most part, the most efficient known flame retardants contain aromatic and/or aliphatic bromine.
The industry is particularly interested in compounds which contain both aromatic bromine and allylic ether. Two examples of this combination are tetrabromobisphenol A allyl ether and tribromophenol allyl ether. However, known examples of these two compounds are unsatisfactory as alternatives to HBCD. These two compounds exhibit unsatisfactory thermal stability, efficiency, and cost. These two compounds also exhibit decreased mechanical properties and deterioration of molecular weight while also presenting concerns regarding their impact on health and the environment.
Recently there have been concerns about the health and environmental impact of some flame retardants including HBCD. Although scientific studies have not necessarily shown significant risks to human health or the environment, there are ongoing reviews by various regulatory agencies that may result in reduced usage of HBCD. In the event that these agencies limit the usage of HBCD, extruded polystyrene foam and expanded polystyrene foam manufacturers may be required to choose an alternative flame retardant, and many may adopt a substitute before any regulatory mandate. Thus, there exists a need for a flame retardant alternative to HBCD that is more environmentally friendly and maintains all of the performance properties of HBCD.